Systems And Methods For Enhanced Recovery Of NGL Hydrocarbons

ABSTRACT

Systems and methods for the enhanced recovery of ethane and heavier hydrocarbons using an absorbing agent. Typical absorbing agents include one or more C3+ alkanes. The systems and methods separate components of a feed gas containing methane and heavier hydrocarbons, which maximizes ethane recovery, without requiring appreciable increases in capital and operating costs, and improves the safety margin with respect to the risk of CO 2  freeze-out.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Divisional Application of and claimspriority to U.S. patent application Ser. No. 13/827,147, titled “Systemsand Methods for Enhanced Recovery of NGL Hydrocarbons”, filed on Mar.14, 2013, which is incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not applicable.

FIELD OF THE INVENTION

The present invention generally relates to systems and methods forenhanced recovery of natural gas liquid (“NGL”) hydrocarbons. Moreparticularly, the present invention relates to the enhanced recovery ofethane and heavier hydrocarbons using an absorbing agent.

BACKGROUND OF THE INVENTION

Natural gas, as a clean energy source, comprises a variety ofhydrocarbon constituents from methane, ethane, propane to much heaviercomponents. Ethane, propane and heavier components are more valuablethan methane. The liquid extraction process is used to recover NGL suchas ethane, propane, and heavier components from the natural gas. A highrecovery of ethane is needed because of its increased demand aspetrochemical feedstock.

Cryogenic expansion using a turbo-expander has become the preferredprocess for high ethane recovery with or without the aid of externalrefrigeration, depending upon the composition (richness) of the gas. Ina conventional turbo-expander process, the feed gas is pre-cooled andpartially condensed by a heat exchanger with other process streamsand/or by external propane refrigeration. The condensed liquid includesless volatile components and is then separated and fed to afractionation column (e.g., a demethanizer), which is operated at amedium or low pressure to recover the heavy hydrocarbon constituentsdesired. The remaining non-condensed vapor portion is subjected toturbo-expansion at a lower pressure, resulting in further cooling andadditional liquid condensation. With the expander discharge pressuretypically the same as the demethanizer pressure, the resultant two-phasestream is fed to the top section of the demethanizer with the coldliquids acting as the reflux to enhance recovery of heavier hydrocarboncomponents. The remaining vapor combines with the column overhead as aresidue gas, which is then recompressed to pipeline pressure after beingheated to recover available refrigeration.

Because the demethanizer described above operates mainly as a strippingcolumn, the expander discharge vapor leaving the column overhead, whichis not subject to rectification, still contains a significant amount ofheavy components. These heavy components could be recovered if they werebrought to a lower temperature, or subject to a rectification step. Thelower temperature option can be achieved by a higher expansion ratioand/or a lower column pressure, but the compression horsepower would betoo high to be economical. Ongoing efforts to achieve a higher liquidrecovery of NGL generally fall into one of the following threecategories: (1) adding a rectification section to reduce the amount ofheavy components escaping through the overhead; (2) providing a colderand leaner reflux stream; and (3) introducing a stripping gas to improvethe separation efficiency of the demethanizer.

In U.S. Pat. Nos. 4,157,904 and 4,278,457, which describe a split-vaporprocess that became the most recognized process for high ethane recoveryusing a rectification section (category (1)), the non-condensed vapor issplit into two portions with the major one passing through aturbo-expander, as usual, while the remaining portion is substantiallysubcooled and introduced near the top of the demethanizer. The colderreflux flow permits an improved ethane recovery in spite of less flowbeing expanded via the turbo-expander. The achievable recovery level,however, is ultimately limited by the composition of the vapor streamused for the top reflux due to equilibrium constraints. Ethane recoveryis therefore, typically 90% when the expansion ratio is high. Multiplevariations, such as U.S. Pat. No. 4,519,824 and U.S. Pat. No. 5,555,748,were proposed later to marginally improve the split-vapor process,however, the energy consumption can increase sharply when higher ethanerecovery is targeted using this split-vapor process.

In category (2), a substantially ethane-free reflux is introduced andpermits in excess of 98% recovery of ethane and heavier components. Thereflux consists of recycling a portion of the residue gas stream that iscondensed and deeply subcooled. However, condensing the recycled residuegas can require a significant amount of refrigeration and compressionpower. The use of a portion of the residue gas compressor discharge forrecycle into a demethanizer is disclosed in U.S. Pat. Nos. 4,687,499 and5,568,737. A variation with a booster compressor is disclosed for a lowresidue gas pressure scenario in the '737 Patent. U.S. Pat. Nos.4,851,020 and 4,889,545 utilize the cold residue gas from thedemethanizer overhead as the recycle stream. This process requires acompressor operating at a cryogenic temperature. Two problems can arisefrom using the residue gas to generate a reflux stream: (1) residue gasbeing mostly methane and lighter components makes condensation difficultand requires significantly higher compression power; and (2) it canincrease the CO2 freeze-up risk in the demethanizer.

In category (3), U.S. Pat. No. 5,992,175 introduces a stripping gasmethod that draws the liquid stream from the lower section of thedemethanizer tower as a refrigerant to chill gas and the returns thecompressed gas to the tower as stripping gas to enhance separation.Since the refrigerant is generated internally, the need for externalrefrigeration system is eliminated. However, the stripping gas methodalone cannot achieve very high ethane recovery.

SUMMARY OF THE INVENTION

The present invention therefore, meets the above needs and overcomes oneor more deficiencies in the prior art by providing systems and methodsfor the enhanced recovery of ethane and heavier hydrocarbons using anabsorbing agent.

In another embodiment, the present invention includes a method forrecovering ethane and heavier hydrocarbons from a hydrocarbon feed gas,which comprises: i) cooling an absorbing agent and an inlet streamcomprising the feed gas in a heat exchanger to produce a cooledabsorbing agent and a chilled inlet stream; ii) separating the chilledinlet stream in a separator to produce a liquid hydrocarbon stream andan overhead vapor stream; iii) combining the cooled absorbing agent witha portion of the overhead vapor stream to form a combined stream; iv)cooling the combined stream in a reflux exchanger to produce a subcooledliquid stream; v) expanding another portion of the overhead vapor streamin an expander to produce a demethanizer feed stream; and

vi) introducing the liquid hydrocarbon stream, the subcooled liquidstream and the demethanizer feed stream into a demethanizer column,wherein the ethane and heavier hydrocarbons are recovered as a bottomproduct in the demethanizer column and methane and lighter hydrocarbonsare recovered as a top product in the demethanizer column.

Additional aspects, advantages and embodiments of the invention willbecome apparent to those skilled in the art from the followingdescription of the various embodiments and related drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described below with reference to theaccompanying drawings in which like elements are referenced with likereference numerals, and in which:

FIG. 1 is a schematic flow diagram illustrating one embodiment of an NGLenhanced recovery system in accordance with the present invention,wherein an overhead vapor stream is enriched with an absorbing agent.

FIG. 2 is a schematic flow diagram illustrating another embodiment of anNGL enhanced recovery system in accordance with the present invention,wherein a residue gas recycle stream is enriched with an absorbingagent.

FIG. 3 is a schematic flow diagram illustrating another embodiment of anNGL enhanced recovery system in accordance with the present invention,wherein an inlet stream comprising feed gas is split and a portion ofthe inlet stream is enriched with an absorbing agent.

FIG. 4 is a schematic flow diagram illustrating another embodiment of anNGL enhanced recovery system in accordance with the present invention,wherein an absorbing agent is used to contact the feed gas in a chilledinlet stream to generate a liquid hydrocarbon stream and an enrichedoverhead vapor stream.

FIG. 5 is a schematic flow diagram illustrating another embodiment of anNGL enhanced recovery system in accordance with the present invention,wherein a recycled absorbing agent stream is produced as a split streamfrom the bottom of a deethanizer column.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The subject matter of the present invention is described withspecificity, however, the description itself is not intended to limitthe scope of the invention. The subject matter thus, might also beembodied in other ways, to include different steps or combinations ofsteps similar to the ones described herein, in conjunction with otherpresent or future technologies. Moreover, although the term “step” maybe used herein to describe different elements of methods employed, theterm should not be interpreted as implying any particular order among orbetween various steps herein disclosed unless otherwise expresslylimited by the description to a particular order. While the followingdescription refers to the oil and gas industry, the systems and methodsof the present invention are not limited thereto and may also be appliedin other industries to achieve similar results.

The following description refers to FIGS. 1-5, which includes systemsand methods for the enhanced recovery of ethane and heavier hydrocarbons(e.g. C2+ and C3+) using an absorbing agent. The systems and methodsseparate components of a feed gas containing methane and heavierhydrocarbons, which maximizes ethane recovery, without requiringappreciable increases in capital and operating costs and improves thesafety margin with respect to the risk of CO₂ freeze-out. As a result,the present invention provides significant improvements in theefficiency and operability of systems and methods for the enhancedrecovery of ethane and heavier hydrocarbons using an absorbing agent.The most preferable absorbing agent for ethane (C2+) recovery consistsof propane and heavier components because the heavier components enhanceabsorption of ethane in the rectification section of the demethanizer.Similarly, the most preferable absorbing agent for propane (C3+)recovery consists of butanes and heavier components. The addition of theabsorbing agent to the reflux raises the critical temperature andpressure of the system, thereby allowing more efficient and/oreconomical separation to be performed. The enriched reflux can becondensed at a lower pressure and thus, reduce compression horsepower.The presence of an absorbing agent in the reflux also enhanceshydrocarbon separation and helps avoid potential solid formationproblems in a cryogenic separation process. To the extent thattemperatures and pressures are used in connection with the followingdescription, those conditions are merely illustrative and are not meantto limit the invention.

Referring now to FIG. 1, a schematic flow diagram illustrates oneembodiment of an NGL enhanced recovery system 100 in accordance with thepresent invention wherein an overhead vapor stream is enriched with anabsorbing agent.

Feed gas, typically comprising a clean, filtered, dehydrated natural gasor refinery fuel gas stream is introduced into the NGL enhanced recoverysystem 100 through inlet stream 2. One or more C3+ components areintroduced into the enhanced recovery system 100 through an absorbingagent 8. The source of the absorbing agent 8 can be an external additiveor, preferably, can be one or more recycled products from fractionationcolumns downstream from a demethanizer column.

The inlet stream 2 and absorbing agent 8 are cooled to a predeterminedtemperature in a heat exchanger 110. The cooling is preferably byindirect heat exchange with at least a residue stream 33, a sidereboiling stream 27, a demethanizer reboiling stream 46, andcombinations thereof to at least partially condense the inlet stream 2.A shortage in the refrigeration, if any, can be effectively supplementedby either the enhanced stripping gas scheme disclosed in U.S. Pat. No.5,992,175, or conventional refrigeration means that are well known inthe art.

A chilled inlet stream 20 from the heat exchanger 110 flows into aseparator 112 where it is separated into vapor and liquid phases. Liquidhydrocarbons collected at the bottom of separator 112 form a liquidhydrocarbon stream 82 that flows into a demethanizer column 118 througha level control valve 135. An overhead vapor stream 30, produced fromseparator 112, is split between line 31 and line 65, which are directedto a reflux exchanger 116 and an expander 115, respectively. Theoverhead vapor stream 30 in line 31 is mixed with a cooled absorbingagent 12 prior to passing through the reflux exchanger 116, wherein thecombined stream 34 is totally condensed and subcooled in the refluxexchanger 116 by indirect heat exchange with an overhead vapor 37 fromthe demethanizer column 118. The overhead vapor stream 30 in line 65 isexpanded in expander 115 and sent to the demethanizer column 118,preferably to a feed location below a subcooled liquid stream 35, as ademethanizer feed stream 80. During the expansion, the temperature ofthe overhead vapor stream 30 in line 65 is lowered and shaftwork isgenerated. This shaftwork is later recovered in a boost compressor 113driven by the expander 115.

The subcooled liquid stream 35 is expanded through an expansion valve133 before entering the top of the demethanizer column 118 as reflux.Ethane and heavier components are recovered in the demethanizer column118 and exit as a bottom liquid stream 66 while methane and lightercomponents are recovered in the demethanizer column 118 and exit as theoverhead vapor 37. The overhead vapor 37 is fed into the refluxexchanger 116, providing refrigeration for condensing and subcoolingcombined stream 34. A residue gas exits the reflux exchanger 116 asresidue stream 33 where it is further warmed to near the temperature ofthe inlet stream 2 in the heat exchanger 110. A warmed residue gasstream 51 from the heat exchanger 110 is sent to the suction end of theboost compressor 113 and exits as a compressed stream 26. Depending uponthe delivery pressure, a residue gas compressor 120 may be needed tofurther compress the compressed stream 26 into a residue gas stream 68for final delivery.

Referring now to FIG. 2, a schematic flow diagram illustrates anotherembodiment of an NGL enhanced recovery system 200 in accordance with thepresent invention, wherein a residue gas recycle stream is enriched withan absorbing agent.

In this embodiment, a residue gas recycle stream 70 is split from theresidue gas stream 68 exiting the residue gas compressor 120. Anabsorbing agent 8, typically comprising one of more C3+ components, ismixed with the residue gas recycle stream 70 to form an enriched residuegas recycle stream 71. The source of the absorbing agent 8 can be anexternal additive or, preferably, can be one or more recycled productsfrom fractionation columns downstream from a demethanizer column.

The inlet stream 2 and the enriched residue gas recycle stream 71 arecooled to a predetermined temperature in the heat exchanger 110. Thecooling is preferably by indirect heat exchange with at least a residuestream 33, a side reboiling stream 27, a demethanizer reboiling stream46, and combinations thereof to at least partially condense the inletstream 2. A shortage in the refrigeration, if any, can be effectivelysupplemented by either the enhanced stripping gas scheme disclosed inU.S. Pat. No. 5,992,175, or conventional refrigeration means that areknown in the art.

A chilled inlet stream 20 from the heat exchanger 110 flows into theseparator 112 where it is separated into vapor and liquid phases. Liquidhydrocarbons collected at the bottom of separator 112 form a liquidhydrocarbon stream 82 that flows into the demethanizer column 118through the level control valve 135. A chilled enriched residue gasrecycle stream 36 leaving the heat exchanger 110 is sent to the refluxexchanger 116, wherein it is totally condensed and subcooled in thereflux exchanger 116 by indirect heat exchange with the overhead vapor37 from the demethanizer column 118. The overhead vapor stream in line65 is expanded in expander 115 and sent to the demethanizer column 118,preferably to a feed location below the subcooled liquid stream 35, as ademethanizer feed stream 80. During the expansion, the temperature ofoverhead vapor stream in line 65 is lowered and shaftwork is generated.This shaftwork is later recovered in a boost compressor 113 driven bythe expander 115.

The subcooled liquid stream 35 is expanded through the expansion valve133 before entering the top of the demethanizer column 118 as reflux.Ethane and heavier components are recovered in the demethanizer column118 and exit as the bottom liquid stream 66 while methane and lightercomponents are recovered in the demethanizer column 118 and exit as theoverhead vapor 37. The overhead vapor 37 is fed to the reflux exchanger116, providing refrigeration for condensing and subcooling the chilledenriched residue gas recycle stream 36. A residue gas exits the refluxexchanger 116 as residue stream 33 where it is further warmed to nearthe temperature of the inlet stream 2 in the heat exchanger 110. Awarmed residue gas stream 51 from the heat exchanger 110 is sent to thesuction end of the boost compressor 113 and exits as the compressedstream 26. Depending upon the delivery pressure, a residue gascompressor 120 may be needed to further compress the compressed stream26 into the residue gas stream 68 for final delivery.

Referring now to FIG. 3, a schematic flow diagram illustrates anotherembodiment of an NGL enhanced recovery system 300 in accordance with thepresent invention, wherein a portion of an inlet stream containing thefeed gas is split and is enriched with an absorbing agent.

In this embodiment, the inlet stream 2 is split between line 4 and line10, wherein the inlet stream 2 in line 10 includes the majority of theinlet stream 2. An absorbing agent 8, typically comprising one of moreC3+ components, is mixed with the inlet stream 2 in line 4 to form anenriched split feed stream 15. Optionally, the enriched split feedstream 15 may be compressed in a compressor 122 to a predeterminedpressure and cooled in a cooler 125 to form an enriched inlet stream 19.The source of the absorbing agent 8 can be an external additive or,preferably, can be one or more recycled products from fractionationcolumns downstream from a demethanizer column.

A portion of inlet stream 2 in line 10 and the enriched inlet stream 19are cooled to a predetermined temperature in the heat exchanger 110. Thecooling is preferably by indirect heat exchange with at least a residuestream 33, a side reboiling stream 27, a demethanizer reboiling stream46, and combinations thereof to at least partially condense the portionof inlet stream 2 in line 10. A shortage in the refrigeration, if any,can be effectively supplemented by either the enhanced stripping gasscheme disclosed in U.S. Pat. No. 5,992,175, or conventionalrefrigeration means that are known in the art.

A chilled inlet stream 20 from the heat exchanger 110 flows intoseparator 112 where it is separated into vapor and liquid phases. Liquidhydrocarbons collected at the bottom of the separator 112 form a liquidhydrocarbon stream 82 that flows into demethanizer column 118 throughlevel control valve 135. A chilled enriched split feed stream 34 aleaving the heat exchanger 110 is optionally sent to another separator114. A bottom liquid separator stream 81 from the another separator 114passes through another level control valve 136 and is mixed with theliquid hydrocarbon stream 82 from the separator 112 before flowing intothe demethanizer column 118 through the level control valve 135.Overhead vapor separator stream 38 from the another separator 114 issent to the reflux exchanger 116, wherein it is totally condensed andsubcooled in the reflux exchanger 116 by indirect heat exchange with theoverhead vapor 37 from the demethanizer column 118. The overhead vaporstream in line 65 is expanded in expander 115 and sent to demethanizercolumn 118, preferably to a feed location below the subcooled liquidstream 35, as a demethanizer feed stream 80. During the expansion, thetemperature of the overhead vapor stream in line 65 is lowered andshaftwork is generated. This shaftwork is later recovered in a boostcompressor 113 driven by the expander 115.

The subcooled liquid stream 35 is expanded through the expansion valve133 before entering the top of the demethanizer column 118 as reflux.Ethane and heavier components are recovered in the demethanizer column118 and exits as the bottom liquid stream 66 while methane and lightercomponents are recovered in the demethanizer column 118 and exits as theoverhead vapor 37. The overhead vapor 37 is fed into the refluxexchanger 116, providing refrigeration for condensing and subcooling theoverhead vapor separator stream 38. A residue gas exits the refluxexchanger 116 as residue stream 33 where it is further warmed to nearthe temperature of the inlet stream 2 in the heat exchanger 110. Awarmed residue gas stream 51 from the heat exchanger 110 is sent to thesuction end of the boost compressor 113 and exits as a compressed stream26. Depending upon the delivery pressure, a residue gas compressor 120may be needed to further compress the compressed stream 26 into aresidue gas stream 68 for final delivery.

Referring now to FIG. 4, a schematic flow diagram illustrates anotherembodiment of an NGL enhanced recovery system 400 in accordance with thepresent invention, wherein an absorbing agent is used to contact thefeed gas in a chilled inlet stream to generate a liquid hydrocarbonstream to a demethanizer column and an enriched overhead vapor stream toan expander.

In this embodiment, the inlet stream 2 and an absorbing agent 8,typically comprising one of more C3+ components, are cooled to apredetermined temperature in a heat exchanger 110. The source of theabsorbing agent 8 can be an external additive or, preferably, can be oneor more recycled products from fractionator columns downstream from ademethanizer column. The cooling is preferably by indirect heat exchangewith at least a residue stream 33, a side reboiling stream 27, ademethanizer reboiling stream 46, and combinations thereof to at leastpartially condense the inlet stream 2. A shortage in the refrigeration,if any, can be effectively supplemented by either the enhanced strippinggas scheme disclosed in U.S. Pat. No. 5,992,175, or conventionalrefrigeration means that are known in the art.

A chilled inlet stream 20 from the heat exchanger 110 flows into thebottom of an absorber 112 a, which may contain one or more mass transferstages. A cooled absorbing agent 12 from the heat exchanger 110 flowsinto the top of the absorber 112 a to primarily recover desired heavycomponents in the form of a liquid hydrocarbon stream 82 a, and enrichthe enriched overhead vapor stream 30 a. The liquid hydrocarbon stream82 a flows into a demethanizer column 118 through a level control valve135. The enriched overhead vapor stream 30 a is split between line 31and line 65, which are directed to a reflux exchanger 116 and anexpander 115, respectively. The enriched overhead vapor stream 30 a inline 31 enters the reflux exchanger 116 wherein it is totally condensedand subcooled in the reflux exchanger 116 by indirect heat exchange withan overhead vapor 37 from the demethanizer column 118. The enrichedoverhead vapor stream 30 a in line 65 is expanded in expander 115 andsent to the demethanizer column 118, preferably to a feed location belowa subcooled liquid stream 35, as a demethanizer feed stream 80. Duringthe expansion, the temperature of the enriched overhead vapor stream 30a in line 65 is lowered and shaftwork is generated. This shaftwork islater recovered in a boost compressor 113 driven by the expander 115.

The subcooled liquid stream 35 is expanded through an expansion valve133 before entering the top of the demethanizer column 118 as reflux.Ethane and heavier components are recovered in the demethanizer column118 and exit as a bottom liquid stream 66 while methane and lightercomponents are recovered in the demethanizer column 118 and exit as theoverhead vapor 37. The overhead vapor 37 is fed to the reflux exchanger116, providing refrigeration for condensing and subcooling the enrichedoverhead vapor stream 30 a in line 31. A residue gas exits the refluxexchanger 116 as residue stream 33 where it is further warmed to nearthe temperature of the inlet stream 2 in the heat exchanger 110. Awarmed residue gas stream 51 from the heat exchanger 110 is sent to thesuction end of the boost compressor 113 and exits as a compressed stream26. Depending upon the delivery pressure, a residue gas compressor 120may be needed to further compress the compressed stream 26 into aresidue gas stream 68 for final delivery.

Referring now to FIG. 5, a schematic flow diagram illustrates anotherembodiment of an NGL enhanced recovery system 500 in accordance with thepresent invention, wherein a recycled absorbing agent stream is producedas a split stream from the bottom of a deethanizer column.

In this embodiment, a residue gas recycle stream 70 is split from theresidue gas stream 68 exiting the residue gas compressor 120. Anabsorbing agent 8, typically comprising one of more C3+ components, ismixed with the residue gas recycle stream 70 to form an enriched residuegas recycle stream 71. The source of the absorbing agent 8 can be anexternal additive or, preferably, can be one or more recycled productsfrom fractionation columns downstream from a demethanizer column.

The inlet stream 2 and the enriched residue gas recycle stream 71 arecooled to a predetermined temperature in the heat exchanger 110. Thecooling is preferably by indirect heat exchange with at least a residuestream 33, a side reboiling stream 27, a demethanizer reboiling stream46, and combinations thereof to at least partially condense the inletstream 2. A shortage in the refrigeration, if any, can be effectivelysupplemented by either the enhanced stripping gas scheme disclosed inU.S. Pat. No. 5,992,175, or conventional refrigeration means that areknown in the art.

A chilled inlet stream 20 from the heat exchanger 110 flows into theseparator 112 where it is separated into vapor and liquid phases. Liquidhydrocarbons collected at the bottom of separator 112 form a liquidhydrocarbon stream 82 that flows into the demethanizer column 118through the level control valve 135. A chilled enriched residue gasrecycle stream 36 leaving the heat exchanger 110 is sent to the refluxexchanger 116, wherein it is totally condensed and subcooled in thereflux exchanger 116 by indirect heat exchange with the overhead vapor37 from the demethanizer column 118. The overhead vapor stream in line65 is expanded in expander 115 and sent to the demethanizer column 118,preferably to a feed location below the subcooled liquid stream 35, as ademethanizer feed stream 80. During the expansion, the temperature ofoverhead vapor stream in line 65 is lowered and shaftwork is generated.This shaftwork is later recovered in a boost compressor 113 driven bythe expander 115.

The subcooled liquid stream 35 is expanded through the expansion valve133 before entering the top of the demethanizer column 118 as reflux.Ethane and heavier components are recovered in the demethanizer column118 and exit as the bottom liquid stream 66 while methane and lightercomponents are recovered in the demethanizer column 118 and exit as theoverhead vapor 37. The overhead vapor 37 is fed to the reflux exchanger116, providing refrigeration for condensing and subcooling the chilledenriched residue gas recycle stream 36. A residue gas exits the refluxexchanger 116 as residue stream 33 where it is further warmed to nearthe temperature of the inlet stream 2 in the heat exchanger 110. Awarmed residue gas stream 51 from the heat exchanger 110 is sent to thesuction end of the boost compressor 113 and exits as the compressedstream 26. Depending upon the delivery pressure, a residue gascompressor 120 may be needed to further compress the compressed stream26 into the residue gas stream 68 for final delivery.

The bottom liquid stream 66 from the demethanizer column 118 enters adeethanizer column 119 through another expansion valve 137. Anethane-rich stream 84 is generated from the top of the deethanizercolumn 119 and a stream 85 containing propane and heavier components isrecovered from the bottom of the deethanizer column 119. The stream 85is split into C3+ product stream 86 and a recycled absorbing agentstream 87 using techniques well known in the art. The recycled absorbingagent stream 87 is transferred by a pump 121 at a predetermined pressurethrough a cooler 138 to form the absorbing agent 8, which is mixed withthe residue gas recycle stream 70 to form the enriched residue gasrecycle stream 71.

EXAMPLE

Table 1 below includes the exemplary feed conditions used for the threesystems compared in Table 2.

TABLE 1 Feed Conditions Temperature, ° C. 4.5 Pressuer, psia 641 MolarFlow (MMSCFD) 1,500 Mass Flow (kg/hr) 1,304,368 Composition (Mol %)Nitrogen 1.21 CO2 0.76 Methane 92.70 Ethane 3.79 Propane 1.07 i-Butane0.15 n-Butane 0.19 i-Pentane 0.05 n-Pentane 0.03 n-Hexane 0.05 n-Heptane0.00 n-Octane 0.00

Table 2 below compares the simulated performance of the split feedcompression system described in U.S. Pat. No. 6,354,105 and twoembodiments of an NGL enhanced recovery system described above inreference to FIGS. 2 and 3. Without an absorbing agent, the split feedcompression system requires a new split feed compressor of 6,359 hpcompared to 4,868 hp for the split feed compression system with anabsorbing agent (FIG. 3). Overall, the total compression power isreduced by 2,141 hp. The residue gas recycle system with an absorbingagent (FIG. 2) only requires a split feed compressor with 3,607 hp.Overall, the total compression power is reduced by 2,755 hp. Thedemethanizer operating pressure is increased to 384 psia to maintain thesame residue gas compression power.

TABLE 2 Split Feed Residue Compression Split Feed Gas w/o CompressionRecycle absorbing w/absorbing w/absorbing agent agent agent DemethanizerPressure, psia 366 366 384 Liquid Recovery Ethane Recovery (%) 80.0 80.080.0 Compression Power, hp Propane Refrigeration 14,324 13,941 14,372Ethane Refrigeration 4,772 4,345 4,513 Residue Gas Compression 38,69038,850 38,898 Split Feed Compression 6,359 4,868 — Residue Gas Recycle —— 3,607 Compression Total Compression (hp) 64,145 62,004 61,390 Δ intotal horsepower = −2,141 −2,755 Δ % in total horsepower = −3.3% −4.3%New Equipment New Compressor Discharge, 1120 985 960 psia New BAHX Duty96.5 95.4 66.4 (MMBtu/h)

While the present invention has been described in connection withpresently preferred embodiments, it will be understood by those skilledin the art that it is not intended to limit the invention to thoseembodiments. It is therefore, contemplated that various alternativeembodiments and modifications may be made to the disclosed embodimentswithout departing from the spirit and scope of the invention defined bythe appended claims and equivalents thereof.

1. A method for recovering ethane and heavier hydrocarbons from ahydrocarbon feed gas, which comprises: cooling an absorbing agent and aninlet stream comprising the feed gas in a heat exchanger to produce acooled absorbing agent and a chilled inlet stream; separating thechilled inlet stream in a separator to produce a liquid hydrocarbonstream and an overhead vapor stream; combining the cooled absorbingagent with a portion of the overhead vapor stream to form a combinedstream; cooling the combined stream into a reflux exchanger to produce asubcooled liquid stream; expanding another portion of the overhead vaporstream in an expander to produce a demethanizer feed stream; andintroducing the liquid hydrocarbon stream, the subcooled liquid streamand the demethanizer feed stream into a demethanizer column, wherein theethane and heavier hydrocarbons are recovered as a bottom product in thedemethanizer column and methane and lighter hydrocarbons are recoveredas a top product in the demethanizer column.
 2. The method of claim 1,wherein the absorbing agent comprises one or more C3+ alkanes.
 3. Themethod of claim 1, wherein the hydrocarbon feed gas comprises methaneand heavier hydrocarbons.
 4. The method of claim 1, wherein theabsorbing agent and the inlet stream are cooled in the heat exchanger byindirect heat exchange with a residue stream, a side reboiling streamand a demethanizer reboiling stream.
 5. The method of claim 1, furthercomprising processing the methane and lighter hydrocarbons in the refluxexchanger, the heat exchanger and a boost compressor to produce aresidue gas stream.